In designing evacuated solar receivers for parabolic trough concentrators, it is desirable to make the receiver or absorber and the evacuated envelope surrounding same as long as possible such that connection and end losses are minimized. By reducing the number of ends, materials requirements are reduced and fabrication is simplified. Further, manifolding is simplified and the significant thermal losses associated therewith can be substantially reduced.
It now appears feasible to manufacture receivers in excess of 20 feet in length. A metal receiver or absorber tube this length would, by its own weight and that of the heat transfer fluid contained therein, deflect much more than would be tolerable to maintain the absorber within the focal line of a parabolic trough. For example, a receiver tube 24 feet long consisting of a steel tube 13/8" O.D. by 0.1" wall thickness supported at its ends would deflect by its own weight about 4" near its center. A glass tube or envelope 21/2" O.D. by 0.1" wall thickness would deflect about 11/2". Even if such bending could be tolerated from a stress point of view, the fact is the absorber or receiver portion of the solar collector would be in contact with the glass envelope and would thus create intolerable thermal losses which the evacuated envelope is designed to reduce. The prior art shows various receiver tube supports. See for example FIGS. 8 and 22 in U.S. Pat. No. 4,133,298. The disadvantages of such a system will become readily apparent hereinafter from the present disclosure.
In addition to the deflection problem mentioned above, the operating temperatures contemplated e.g. 650.degree. F., means that, a steel receiver tube or absorber would expand linearily about 11/2" through an extreme excursion from cold to hot in the thermal cycle. The support for the absorber therefor must be able to accommodate the movement between the absorber and the glass envelope. The glass also expands but to a much lesser degree since the thermal expansion is much lower and the temperature for the glass would not reach the temperature of the absorber (probably no more than 250.degree. F.).
The support for an extremely long receiver tube therefore must be substantial to provide the support necessary considering the greater weight and amount of deflection and must also be capable of accommodating the linear expansion of the receiver tube while at the same time protecting the inside surface of the glass envelope or housing from abrasion over a contemplated useful life of twenty years.